Noise filter

ABSTRACT

A noise filter  10  of the present invention includes an inductor  71  and a resistor  11  connected in parallel with each other. A power supply frequency current bypasses the resistor  11  but passes through the inductor  71.  A high frequency noise current including a resonance frequency current, on the other hand, bypasses the inductor  71  but is dissipated at the resistor  11.  Therefore, the noise filter  10  does not accumulate noise power and so does not suffer from a problem due to power releasing. The resonance frequency current caused by combination of inductance of the inductance and earth capacitance in the noise filter  10  is also dissipated at the resistor  11,  thus giving rise to no problem of its own.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a noise filter provided with aninductor which is attached to a conductor such as a power supply line ora ground line, suppresses noise induced on the conductor.

[0003] 2. Description of the Related Art

[0004] A conventional noise filter comprises an inductor and acapacitor, having a function to discriminate a frequency of an undesiredsignal called “noise” from others to thereby pass only the signal ofpower supply frequencies etc. to the side of equipment. The noise has afrequency of, for example, 10 kHz or higher. The commercial power supplyhas a frequency of 50 or 60 Hz in Japan. Note here that in thisspecification the term “equipment” refers to an electric apparatus, anelectric device, etc. in general.

[0005]FIG. 6 is a circuit diagram showing the conventional noise filter.The following will describe the noise filter with reference to FIG. 6.

[0006] A noise filter 70 shown in FIG. 6[1] is a two-terminal type noisefilter for use on the ground line, comprised of one inductor 71. Oneterminal 72 of the two is connected to the ground and the other terminal73, to the equipment. Also, inductance of the inductor 71 is set so thata power supply frequency current including a leakage current may bedischarged from the equipment to the ground and also a noise currentinduced on the ground line may be blocked.

[0007] A noise filter 80 shown in FIG. 6 [2] is a four-terminal typenoise filter for use on the power supply line, comprised of fourinductors 81-84 and three capacitors 85-87. Respectively, one terminals88 and 89 are connected to the power supply and the other terminals 90and 91, to the equipment. Also, the inductance of the inductors 81-84and the static capacitance of the capacitors 85-87 are set so that thepower supply frequency current may be flown from the power supply to theequipment and the noise current induced on the power supply line may beblocked.

[0008] The conventional noise filters, however, have the followingproblems.

[0009] A noise power referred to as “noise” is induced not only in asteady-state current on the power supply line or the ground line butalso in a pulse-state current with non-periodically. In such a case, theinductors and capacitors of the noise filter, which accumulate powerthereon owing to magnetic and electric fields respectively, release theaccumulated power when noise power stops flowing thereto. As such, thisreleased power may cause the equipment to fail or temporarilydeteriorate in functioning.

[0010] In order to reduce the noise current induced on the ground line,an inductor such as a coil is used. In particular, large equipmentconnected to the ground line has a considerably large capacitance withrespect to the ground, which may sometimes be combined with theinductance of the inductor to give rise to series resonance. A resultantresonance frequency current may flow into the equipment, thus generatinga failure due to noise.

SUMMARY OF THE INVENTION

[0011] In view of the above, it is an object of the present invention toprovide a noise filter which can solve a problem due to power releasedtherefrom and a problem due to a resonance frequency current caused bycombination of the inductance and the earth capacitance thereof.

[0012] The present inventor greatly engaged in researches to solve theseproblems and found that “a problem due to power released from a noisefilter occurs if it does not have a function to dissipate a noisecurrent (transform it into thermal energy)”. The present invention isbased on this finding. That is, by connecting a resistor to the noisefilter in parallel with the inductor, the noise current can bedissipated at the resistor. Also, in such a circuit configuration, theresistor will act also to attenuate a series resonance current generatedby combination of the capacitance-ground and the inductance. It isdetailed as follows.

[0013] The noise filter according to present invention is attached to aconductor such as a power supply line or a ground line and which isprovided with an inductor for suppressing noise induced on saidconductor, wherein a resistor is connected in parallel with saidinductor. In this configuration, the power supply current having lowfrequency passes through the inductor without power loss while bypassingthe resistor. On the other hand, high frequency noise current includingthe resonance frequency current does not pass the inductor and the highfrequency noise current is consumed in the resistor. Accordingly, sincean electric power caused by the noise is not accumulated in the noisefilter, the problem caused by discharging of the electric power is notoccurred. Also, since the resonance frequency current caused by thenoise filter and the earth capacitance is consumed in the resistor, theproblem caused by the resonance frequency current is not occurred.

[0014] The noise filter may be configured that a parallel circuitconsisting of the inductor and the resistor is attached to one groundline, with one terminal thereof connected to the ground and the otherterminal thereof connected to equipment. The noise filter of thisconfiguration is called a ground line noise filter. The number ofinductors and resistors may be single or plural. When there are aplurality of inductors, a configuration of at least one inductor beingconnected in parallel with the resister may be acceptable.

[0015] Assuming an angular frequency of a power supply current to beωp[rad], a lower limit angular frequency of a noise current to beωn[rad], inductance of said inductor to be L[H], and resistance of saidresistor to be R[Ω] preferably a relationship of 10 (ωp·L)<R< (ωn·L)/10is established, and more preferably a relationship of 100(ωp·L)<R<(ωn·L) /100 is established, and most preferably a relationshipof 1000 (ωp·L)>R>(ωn·L)/1000 is established. By thus narrowing a rangeof the value of R, it is possible to obtain well balancedcharacteristics that an attenuation quantity at ωp is smallappropriately and that at ωn is large appropriately.

[0016] Also, a relationship of (ωn·L)/R≧1/(2 ωn) may be established. Inthis case, the power dissipated at the resistor exceeds the poweraccumulated on the inductor.

[0017] When the power supply line is provided more than one, the noisefilter may be configured that the parallel circuit consisting of theinductor and the resistor is provided to each of the power supply linesand a capacitor is provided between the power supply lines. The noisefilter of this configuration is called a noise filter for use on a powersupply line. The number of inductors and resistors may be single orplural. When there are a plurality of inductors, a configuration of atleast one inductor being connected in parallel with the resister may beacceptable.

[0018] The resistor may be a variable resistor. As the noise filter isattached to various units of equipment, they have fluctuations in valueof the earth capacitance. The resultant fluctuations in resonancefrequency can also be accommodated properly by changing the resistanceof the variable resistors.

[0019] The noise filter may have a configuration that the inductor is atoroidal coil, the resistor is a variable resistor, the parallel circuitconsisting of the toroidal coil and the variable resistor is housed in aframe, the variable resistor is arranged in a space surrounded by aninner peripheral wall of the toroidal coil, and resistance varying meansfor varying resistance of the variable resistor is provided at such aposition as to be able to be operated from an outside of the frame. Byoperating the resistance varying means from the outside of the frame,fluctuations in value of the earth capacitance can be easilyaccommodated. Also, since the variable resistor is arranged in the spacesurrounded by the inner peripheral wall of the toroidal coil, the spacein the frame can be utilized effectively.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1[1] is a circuit diagram for showing a noise filteraccording to a first embodiment of the present invention;

[0021]FIG. 1[2] is a circuit diagram for showing the noise filteraccording to a second embodiment of the present invention;

[0022]FIG. 2 is a circuit diagram for showing a state where the noisefilter of FIG. 1 is used;

[0023] Of FIGS. 3 for showing attenuation characteristics of the noisefilter of FIG. 1, FIG. 3[1] shows a state where R=

, FIG. 3[2] shows a state where R=10 kΩ, FIG. 3[3] shows a state whereR=1 kΩ, and FIG. 3[4] shows a state where R=0.1 kΩ;

[0024]FIG. 4[1] is a circuit diagram for showing the noise filteraccording to a third embodiment of the present invention;

[0025]FIG. 4[2] is a circuit diagram for showing the noise filteraccording to a fourth embodiment of the present invention;

[0026]FIG. 5 is a perspective view for showing the noise filteraccording to a fifth embodiment of the present invention; and

[0027]FIG. 6[1] is a circuit diagram for showing a first example of aconventional noise filter and FIG. 6[2], a second example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

[0028]FIG. 1[1] is a circuit diagram for showing a noise filteraccording to a first embodiment of the present invention, and FIG. 2 isa circuit diagram for showing a state where the noise filter of FIG. 1is used. The following will describe the present invention withreference to these figures. The same components in these figures asthose in FIG. 6[1] are indicated by the same reference numerals andtheir explanation is omitted.

[0029] A noise filter 10 of this embodiment comprises an inductor 71 anda resistor 11 connected in parallel with each other. The parallelcircuit comprised of the inductor 71 and the resistor 11 has its oneterminal 72 connected to the ground and the other terminal 73 connectedto equipment 12. As such, the noise filter 10 is a 2-terminal type onefor use on a ground line.

[0030] In this parallel circuit, a power-frequency current bypasses theresistor 11 but passes through the inductor 71 with no loss. Ahigh-frequency noise current including a resonance frequency current, onthe other hand, passes not through the inductor 71 but is dissipated atthe resistor 11. Therefore, the noise filter 10 does not accumulatenoise power and so has no problem caused by releasing of power. Aresonance frequency current caused by combination of inductance of thenoise filter 10 and earth capacitance C is also dissipated at theresistor 11, thus giving no problem of this kind.

[0031] One such noise filter 10 is used on one conductor to thereby havethe two terminals 72 and 73, so that it cannot use but the inductor 71.In this configuration, assuming inductance of the inductor 71 to be Land an angular frequency to be ω, the reactance is ωL. The reactance dueto the earth capacitance C of the equipment 12, on the other hand, is1/ωC, which resonates to ωL in series, thus generating a noise current.

[0032] In this embodiment, the inductor 71 is connected parallel withthe resistor 11. As such, a low-frequency current such as a power supplycurrent or a ground current in short-circuiting passes as it is throughthe inductor 71 with low reactance, while a noise current having ahigher frequency component passes through the resistor 11 to have itspower dissipated thereat.

[0033] That is, assuming resistance of the resistor 11 to be R,impedance Z of the noise filter 10 is given as follows:

Z=[1/{R ²+(ωL)² }]·{R(ωL)² +jR ²(ωL)}  (1)

[0034] If ωL<<R,

Z=[1/{1+(ωL/R)²}]·{(ωL)² /R+jωL}≈jω  (2)

[0035] If ωL>>R,

Z=[1/{1+(R/ωL)² }]·{R+jR(R/ωL)}≈R   (3)

[0036] As is clear from Equation (2), since the impedance of the noisefilter 10 is Z≈jωL, the lower frequency current (power supply current)passes through the noise filter 10 with almost no loss. The highfrequency current (noise current) including a resonance frequency one,on the other hand, is dissipated at the noise filter 10 because itsimpedance is Z≈R as is clear from Equation (3).

[0037] Assuming an angular frequency of the power supply current to beωp [rad], a lower limit angular frequency of the noise current to be ωn[rad], inductance of the inductor 71 to be L [H], and resistance of theresistor 11 to be R[Ω], the following equation must be established as isclear from Equations (2) and (3):

(ωp·L)<<R<<(ωn·L)   (4)

[0038] As for a relationship of (ωp·L)<<R on the left hand side ofEquation (4), R should preferably be larger than (ωp·L) as much aspossible. As for a relationship of R<<(ωn·L) on the right hand side ofEquation (4), on the other hand, R should preferably be smaller than(ωn·L) as much as possible. To trade off these two relationships,preferably the following equation is established:

10(ωp·L)<R<(ωn·L)/10   (5)

[0039] More preferably the following equation is established:

100(ωp·L)<R<(ωn·L)/100   (6)

[0040] Most preferably the following equation is established:

1000(ωp·L)<R<(ωn·L)/1000   (7)

[0041] By thus narrowing a range of the value of R, it is possible toobtain well balanced characteristics that an attenuation quantity at ωpis small appropriately and that at ωn is large appropriately.

[0042] The following will further detail an appropriate relationshipamong ωn, L, and R with reference to FIG. 2.

[0043] A noise current In due to a noise power supply voltage Vn flowsthrough the ground line to the side of the equipment 12, thus givingrise to a failure there. This failure of the equipment 12 can beprevented by the noise filter 10, which transforms part of the noisecurrent In into heat at the resistor 11. If it is not significant indegree, the failure of the equipment 12 may sometimes be recovered onlyby dissipating a little part of the noise current In at the resistor 11.Therefore, basically the value of R is not limited.

[0044] Assuming a voltage across the noise filter 10 to be Vf, a currentflowing through the inductor 71 to be I1, and a current flowing throughthe resistor 11 to be Ir, the following equations are given:

I1=Vf/(ωn·L)   (8)

Ir=Vf/R   (9)

[0045] As such, a power W1 accumulated on the inductor 71 is given by:

W1=L·I1 ²/2=Vf²/(2ωn²·L)   (10)

[0046] Then, a power Pr dissipated at the resistor 11 is given by:

Pr=Ir²·R=Vf²/R   (11)

[0047] In this case, preferably the power Pr dissipated at the resistor11 exceeds at least the power W2 accumulated on the inductor 71, that isPr≧W1. Therefore, the following equation is established from Equations(10) and (11):

W1/Pr=R/(2ωn²·L)≦1   (12)

∴(ωn·L)/R≧1/(2ωn)   (13)

[0048] It is here assumed that L=3[mH] and ωn=2π×100 [rad], for example.The parameter ωn represents a second harmonic of a commercial powersupply with a frequency of 50 [Hz]. In this case, the following equationis given:

(2π×100×0.003)/R=0.6π/R≧1/(4π×100)

∴R≦240π²≈2.37[kΩ]  (14)

[0049] That is, the noise filter 10 having the resistor 11 with a valueof R that satisfies Equation (14) can essentially cancel the noise whichis not lower in frequency than the second harmonic of the power supplyfrequency.

[0050] The following will describe attenuation characteristics withreference to FIGS. 1[1], 2, and 3.

[0051] In the noise filter 10, the inductance of the inductor 71 is L=3mH (100 turns) and the earth capacitance of the equipment 12 is C=800pF. The resistance R of the resistor 11 is

(that is there exists no resistor 11) in FIG. 3[1], 10 kΩ in FIG. 3[2],1 kΩ in FIG. 3[3], and 0.1 kΩ in FIG. 3[4].

[0052] The noise filter of FIG. 3[1] has the same configuration as thatof the conventional noise filter 70 (see FIG. 6[1]), exhibiting a largedrop in attenuation quantity in a region of harmonics including aresonance frequency. The noise filters of FIGS. 3[2]-3[4], on the otherhand, have the same configuration as that of the noise filter 10 of thisembodiment. They exhibit such characteristics that a drop in attenuationquantity decreases as R decreases from

to 10 kΩ and then to 1 kΩ and is eliminated at R=0.1 kΩ.

Second Embodiment

[0053]FIG. 1[2] is a circuit diagram for showing the noise filteraccording to a second embodiment of the present invention. The followingwill describe with reference to FIG. 1[2] . The same components in thisfigure as those in FIG. 6[2] are indicated by the same referencenumerals and their explanation is omitted.

[0054] A noise filter 20 of this embodiment comprises a parallel circuitconsisting of the inductor 81 and a resistor 21 and another parallelcircuit consisting of the inductor 83 and a resistor 23, which parallelcircuits are provided in series to one power supply line, and a parallelcircuit consisting of the inductor 82 and a resistor 22 and anotherparallel circuit consisting of the inductor 84 and a resistor 24, whichparallel circuits are provided in series to the other power supply line,in such a configuration that capacitors 85, 86, and 87 are connectedparallel between these two power supply lines. The noise filter 20 thusprovides a four-terminal noise filter for use on a power supply line.Note here that at least one of the inductor 81-84 may be provided with aresistor.

[0055] A power-frequency current bypasses the resistors 21-24 but passesthrough the inductors 81-84 with no loss. A high-frequency noise currentincluding a resonance frequency current, on the other hand, passes notthrough the inductors 81-84 but is dissipated at the resistors 21-24.Therefore, the noise filter 20 does not accumulate noise power and sohas no problem caused by releasing of power. A resonance frequencycurrent caused by combination of inductance of the noise filter 20 andearth capacitance C is also dissipated at the resistors 21-24, thusgiving no problem of this kind.

[0056] A noise current induced on a conductor such as the power supplyline or the ground line supplies an unnecessary noise power to theequipment, thus causing it to fail or deteriorate in functioning. Thisnoise current is transmitted in two transmission modes: a normal modeand a common mode. In the normal mode, noise currents flow through twoconductors such as the power supply line with the same amplitude and inthe opposite directions. In the common mode, noise currents flow throughtwo conductors such as the power supply line with the same amplitude andin the same direction.

[0057] Generally, noise currents causing a failure to the equipment aregiven in a mutually superimposed form of these two transmission modes.Of these, the noise current given in the normal mode has been suppressedwith such a four-terminal noise filter as shown in FIG. 6[2]. Itsconfiguration, however, has inductors and capacitors only and so has nofunction to decrease the noise current by dissipating it as heat. Tosolve this problem, the noise filter 20 of this embodiment connects theresistors 21-24 to the inductors 81-84 of the conventional four-terminalnoise filter respectively in parallel with each other, thus having thefunction to decrease the high frequency noise current by losing it inheat. This configuration can also be expected to give almost the sameeffects of noise current attenuation as those by the above-mentionedtwo-terminal noise filter shown in FIG. 1[1]

Third Embodiment

[0058]FIG. 4[1] is a circuit diagram for showing the noise filteraccording to a third embodiment of the present invention. The followingwill describe with reference to FIG. 4[1]. The same components in thisfigure as those in FIG. 1[1] are indicated by the same referencenumerals and their explanation is omitted.

[0059] A noise filter 30 of this embodiment has replaced the resistor 11(see FIG. 1[1]) of the first embodiment with a variable resistor 31. Asthe noise filter 30 is attached to various units of equipment, they havefluctuations in value of the earth capacitance C. The resultantfluctuations in resonance frequency can also be accommodated properly bychanging the resistance of the variable resistor 31.

Fourth Embodiment

[0060]FIG. 4[2] is a circuit diagram for showing the noise filteraccording to a fourth embodiment of the present invention. The followingwill describe with reference to FIG. 4 [2]. The same components in thisfigure as those in FIG. 1[2] are indicated by the same referencenumerals and their explanation is omitted.

[0061] A noise filter 40 of this embodiment has replaced the resistors21-24 (see FIG. 1[2]) of the second embodiment with variable resistors41-44 respectively. As the noise filter 40 is attached to various unitsof equipment, they have fluctuations in value of the earth capacitanceC. The resultant fluctuations in resonance frequency can also beaccommodated properly by changing the resistance of the variableresistors 41-44. Note here that at least one of the inductors 81-84 maybe provided with a variable resistor. Also, at least one of theresistors 21-24 (see FIG. 2[1]) of the second embodiment may be replacedby a variable resistor.

Fifth Embodiment

[0062]FIG. 5 is a perspective view for showing the noise filteraccording to a fifth embodiment of the present invention. The followingwill describe with reference to FIG. 5. This embodiment employs the samecircuit as that of FIG. 4[1], so that the same components in FIG. 5 asthose in FIG. 4[1] are indicated by the same reference numerals andtheir explanation is omitted.

[0063] A noise filter 50 of this embodiment comprises a toroidal coil 71and a variable resistor 31 which are connected in parallel with eachother. A parallel circuit consisting of the toroidal coil 71 and thevariable resistor 31 has its one terminal connected through a connector51 to the ground and the other terminal connected through a connector 52to the equipment. The noise filter 50 thus provides a two-terminal noisefilter for use on the ground line. The parallel circuit consisting ofthe toroidal coil 71 and the variable resistor 31 is housed in a frame53. The frame 53 is made of, for example, metal such as aluminum orconductive plastic.

[0064] Also, the variable resistor 31 is arranged at the center of thetoroidal coil 71. That is, the variable resistor 31 is housed at thecenter of the toroidal coil 71, to effectively utilize the space in theframe 53.

[0065] Further, a rotary shaft (resistance varying means) 54 for varyingthe resistance of the variable resistor 31 is disposed at such aposition that it can be operated from the outside of the frame 53. Inthis configuration, by operating the rotary shaft from the outside ofthe frame 53, fluctuations in value of the earth capacitance C can beeasily accommodated. Specifically, since the frame 53 is provided with atranslucent hole 55 formed therein, a standard screwdriver can beinserted through it to rotate the rotary shaft 54 easily.

[0066] As the noise filter 50 is attached to various units of equipment,they have fluctuations in value of the earth capacitance C. As such,after the noise filter 50 is attached to the equipment, the rotary shaft54 can be operated to obtain desired attenuation characteristics.

[0067] It should be appreciated that of course the present invention isnot limited to the above-mentioned first through fifth embodiments. Forexample, the noise filter of any one of the first through fourthembodiments may be arranged in parallel with each other as many as Nnumber and housed in one frame made of metal or nonmetal material tothereby provide a noise filter having 2N or 4N number of terminals.Here, N is two or larger integer.

[0068] The noise filter of the present invention has such a simpleconfiguration that an inductor is connected with a resistor in parallelwith each other in a conventional noise filter, so that a high frequencynoise current including a resonance frequency current bypasses theinductor and is dissipated at the resistor, thus making it possible toprevent the equipment from malfunctioning due to power releasing andalso suppress a resonance frequency current due to the earth capacitanceof the equipment.

[0069] Also, the noise filter according to present invention, since aparallel circuit consisting of said inductor and said resistor isattached to one said ground line, with one terminal thereof connected tothe ground and the other terminal thereof connected to equipment, noisefilter of present invention can be used for ground line.

[0070] Also, the noise filter according to present invention, assumingan angular frequency of a power supply current to be ωp[rad], a lowerlimit angular frequency of a noise current to be ωn[rad], inductance ofsaid inductor to be L[H], and resistance of said resistor to be R[Ω],preferably a relationship of 10(ωp·L)<R<(ωn·L)/10 is established, andmore preferably a relationship of 100(ωp·L)<R<(ωn·L)/100 is established,and most preferably a relationship of 1000(ωp·L)<R<(ωn·L)/100 isestablished. Accordingly, it is possible to obtain well balancedcharacteristics that an attenuation quantity at ωp is smallappropriately and that at ωn is large appropriately.

[0071] Also, a relationship of (ωn·L)/R≧1/(2ωn) may be established. Inthis case, the power dissipated at the resistor exceeds the poweraccumulated on the inductor.

[0072] When the power supply line is provided more than one, the noisefilter may be configured that the parallel circuit consisting of theinductor and the resistor is provided to each of the power supply linesand a capacitor is provided between the power supply lines. The noisefilter of this configuration can be used for power supply line.

[0073] The resistor may be a variable resistor. As the noise filter isattached to various units of equipment, they have fluctuations in valueof the earth capacitance. The resultant fluctuations in resonancefrequency can also be accommodated properly by changing the resistanceof the variable resistors.

[0074] The noise filter may have a configuration that a resistancevarying means for varying resistance of the variable resistor isprovided at such a position as to be able to be operated from an outsideof the frame. Accordingly, by operating the resistance varying meansfrom the outside of the frame, fluctuations in value of the earthcapacitance can be easily accommodated. Further, the variable resistoris arranged in a space surrounded by an inner peripheral wall of thetoroidal coil, thus the space in the frame can be utilized effectively.Accordingly, miniaturization and lightening of the noise filter can beachieved.

[0075] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristic thereof. Thepresent embodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended Claims rather than by the foregoingdescription and all changes which come within the meaning and range ofequivalency of the Claims are therefore intended to be embraced therein.

[0076] The entire disclosure of Japanese Patent Application No.2001-391303 (Filed on Dec. 25, 2001) including specification, Claims,drawings and summary are incorporated herein by reference in itsentirety.

What is claimed is:
 1. A noise filter which is attached to a conductorsuch as a power supply line or a ground line and which is provided withan inductor for suppressing noise induced on said conductor, wherein aresistor is connected in parallel with said inductor.
 2. The noisefilter according to claim 1, wherein a parallel circuit consisting ofsaid inductor and said resistor is attached to one said ground line,with one terminal thereof connected to the ground and the other terminalthereof connected to equipment.
 3. The noise filter according to claim1, wherein assuming an angular frequency of a power supply current to beωp[rad], a lower limit angular frequency of a noise current to beωn[rad], inductance of said inductor to be L[H], and resistance of saidresistor to be R[Ω], a relationship of 10(ωp·L)<R<(ωn·L)/10 isestablished.
 4. The noise filter according to claim 1, wherein assumingan angular frequency of the power supply current to be ωp[rad], thelower limit angular frequency of the noise current to be ωn[rad], theinductance of said inductor to be L[H], and the resistance of saidresistor to be R[Ω], a relationship of 100 (ωp·L)<R<(ωn·L)/100 isestablished.
 5. The noise filter according to claim 1, wherein assumingan angular frequency of the power supply current to be ωp[rad], thelower limit angular frequency of the noise current to be ωn[rad], theinductance of said inductor to be L[H], and the resistance of saidresistor to be R[Ω], a relationship of 1000(ωp·L)<R<(ωn·L)/1000 isestablished.
 6. The noise filter according to claim 1, wherein assumingthe lower limit angular frequency of the noise current to be ωn[rad],the inductance of said inductor to be L[H], and the resistance of saidresistor to be R[Ω], a relationship of (ωn·L)/R≧1/(2ωn) is established.7. The noise filter according to claim 1, wherein said power supply lineis provided more than one, so that the parallel circuit consisting ofsaid inductor and said resistor is provided to each of said power supplylines and a capacitor is provided between said power supply lines. 8.The noise filter according to claim 1, wherein said resistor is avariable resistor.
 9. The noise filter according to claim 1, whereinsaid inductor is a toroidal coil, said resistor is a variable resistor,the parallel circuit consisting of said toroidal coil and said variableresistor is housed in a frame, said variable resistor is arranged in aspace surrounded by an inner peripheral wall of said toroidal coil, andresistance varying means for varying resistance of said variableresistor is provided at such a position as to be able to be operatedfrom an outside of said frame.